Electrical steel sheets are steel sheets having excellent magnetic properties with high magnetic permeability and small core loss. For example, grain oriented electrical steel sheets are often used in cores of transformers. Alternating current flowing through an electric wire wound around a core of a transformer generates an alternating magnetic field inside the core. In general, if an alternating magnetic field is applied in a steel sheet, eddy current loss and hysteresis loss are generated. Electrical steel sheets are demanded to have such eddy current loss and hysteresis loss reduced therein.
The generation of eddy current loss upon the application of an alternating magnetic field to a steel sheet is unavoidable, and the greater the frequency is, the greater the eddy current loss will be. However, a width of a magnetic domain of a steel sheet is one factor influencing this eddy current loss, and the narrower this width is, the more the eddy current loss is able to be reduced. Thus, magnetic properties of a steel sheet and a shape of the magnetic domain are known to be very deeply related to each other.
Accordingly, in a manufacturing process of grain oriented electrical steel sheets, in order to reduce the eddy current loss, a method of refining magnetic domains (magnetic domain refining process) is performed. A magnetic domain of a grain oriented electrical steel sheet extends in a rolling direction, and by adding a strain or forming a groove in a direction intersecting this magnetic domain, the magnetic domain is able to be refined (for example, see Patent Literature 1). A method of giving a thermal strain by irradiation with, for example, laser, electron beams, or the like, in the direction intersecting the magnetic domain, is known as a method of adding the strain. A technique is known, which is for observing a magnetic domain structure in order to inspect whether or not this magnetic domain refining process has been performed appropriately (see Patent Literature 2).